Emulated devices have old User Agents

[arjunyel]

Steps to reproduce

Tell us about your environment:

• Puppeteer version: ^1.11.0-next.1548707281525
• Platform / OS version: macOS Mojave
• URLs (if applicable): https://github.com/GoogleChrome/puppeteer/blob/master/DeviceDescriptors.js
• Node.js version: 11.8.0

What steps will reproduce the problem?

Please include code that reproduces the issue.

1. Try to run an app that uses https://browser-update.org/
2. Emulate device like page.emulate(devices['Pixel 2 XL']); and take a screenshot
3. Screenshot displays warning from browser-update "Your web browser (Chrome 69) is out of date"

What is the expected result?
Expected result is the user agent is either the version of chromium or at least the current stable version available on mobile (e.g. Chrome 72 starting tomorrow);

What happens instead?
Instead all the user agents have chrome 69 hardcoded https://github.com/GoogleChrome/puppeteer/blob/master/DeviceDescriptors.js